Researchers at Technion – Israel Institute
of Technology (Haifa, Israel) and the University of Toronto (Toronto, ON, Canada)
are using the subtle phase and frequency
cues from the myriad light sources
powered by our alternating-current (AC)
electrical grid not only to understand
power distribution and different light-source characteristics, but to perform
nocturnal, high-dynamic-range, low-light
imaging of artificially lit scenes. 1

Different light sources, including
ordinary bulbs, neon, LEDs, and fluorescent lights, all have a unique flicker signature with changes in intensity and
spectral power distribution because of
the grid’s AC, so this information—often
discarded for causing unnatural-look-ing colors in photos and temporal aliasing
in videos—can be used to develop a
time-varying model and, through subsequent coded-exposure computational
imaging, reveal details about a nighttime scene that would not be obvious to
the ordinary observer. Furthermore, this
AC-enabled camera, or “ACam,” can be
used internationally under 110 or 220 V
standard lighting conditions.

The ACam prototype

The 50 or 60 Hz nominal AC frequency of a light source creates a spatially varying, quasiperiodic signal in the
image: shadows and specularities dim or
brighten, according to the AC phase and
the type of each light source. To adequately capture the hidden information in
an AC-lit scene, the ACam camera uses
a coded-exposure imaging technique
that acquires high-dynamic-range (HDR)
images that correspond to fractions of
the AC cycle by capturing long exposures
that synchronize with the AC signal by
masking and unmasking pixels at 2. 7 kHz.

Using a series of equations thattake into account the illuminance,frequency, flicker, color, and otherattributes of various artificial sources,the researchers created a databaseof electric lights called DELIGHT thatlooks at a variety of light sources(including high-pressure sodium, metalhalide, mercury, fluorescent, and LED